set pager in foc

[Stefan Kalkowski]

Hi Chen,

On 22.06.2011 02:54, Chen Tian wrote:
> Thanks for the reply, two more questions. 
> 
> First, why _pager is considered global? Isn't it constructed for each thread
> as a client of an RM session?

Yeah you're right. That's why I put global pager in double quotes. With
global pager I meant that for almost all applications in
Genode/Fiasco.OC there is only one pager thread in core, like you've
stated before. So the local member _pager is mostly the same capability
for all different thread objects (except for instance L4Linux).

> Second, the pager thread whose entry() is defined in pager.cc should be the
> one that receives the page fault request from the kernel. But how is this
> thread registered into kernel so that the kernel knows its existence?

At first, the pager thread is a normal thread which starts up and waits
for IPC messages. In Fiasco.OC one can send IPC messages to a thread
only, when owning a capability for that thread. Whenever a thread raises
a pagefault, the kernel delivers an IPC-message to the pager of that
thread. Therefore the kernel uses a capability, that was registered as
the thread's pager thread beforehand. This registration of the pager
capability is done via the l4_thread_control_* system call.

Does that clarify your question?

regards Stefan

> 
> Thanks for the help.
> 
> Best,
> Chen
> 
> -----Original Message-----
> From: Stefan Kalkowski [mailto:stefan.kalkowski at ...1...] 
> Sent: Monday, June 20, 2011 7:10 AM
> To: Genode OS Framework Mailing List
> Subject: Re: set pager in foc
> 
> Hello Chian,
> 
> On 18.06.2011 01:38, Chen Tian - SISA wrote:
>> Hello,
>>
>>  
>>
>> After the set_pager function of a thread is called through a
>> cpu_session, the _pager  of that thread is set and used when the actual
>> platform thread starts. 
> 
> That's true at least for the implementation of the rm_session interface
> in core with respect to Genode/Fiasco.OC.
> 
>> But there is only one thread (pager) to resolve
>> all page faults for all threads.
> 
> This is almost true for the Fiasco.OC version of Genode. Like Norman
> explained in a previous mail, other platforms don't necessarily have
> only one pager. In the current implementation for Fiasco.OC this is true
> for normal Genode applications. L4Linux for example pages its VCPUs, and
> applications itself. A VCPU (some kind of special thread concept in
> recent Fiasco.OC revisions) is constructed by Genode's core process, but
> afterwards the capability slot containing the global pager capability
> gets overmapped to contain a special pager thread, which is running in
> the L4Linux-kernel protection-domain.
> 
>> I think the pager mapping  happened in
>> platform_thread::start (platform_thread.cc:40-47). Can somebody please
>> explain to me the meaning of this l4_task_map call?
> 
> In Fiasco.OC a pager gets assigned to a thread by choosing a capability
> slot in the capability space of the target protection domain, wherein
> the new thread will run. The capability in that slot should point to the
> pager thread. That why we first map the capability of the pager (which
> was set by the cpu_session interface first) into the capability space of
> the target protection domain. This is what the following snipet does:
> 
>   l4_task_map(_platform_pd->native_task(), L4_BASE_TASK_CAP,
>               l4_obj_fpage(_pager->cap().dst(), 0, L4_FPAGE_RWX),
>               _remote_pager_cap | L4_ITEM_MAP);
> 
> After that, you instruct the kernel to use the capability slot, we just
> filled with life, as pager and exception-handler:
> 
>   l4_thread_control_start();
>   l4_thread_control_pager(_remote_pager_cap);
>   l4_thread_control_exc_handler(_remote_pager_cap);
>   l4_thread_control_bind(_utcb, _platform_pd->native_task());
> 
>> I suspect that
>> “_remote_pager_cap” is corresponding to  the global pager, but cannot
>> find any evidence.  Thanks a lot.
> 
> The '_remote_pager_cap' denotes the capability slot in the thread's
> capability space, whereby '_pager' is the capability slot in core's
> capability space, that gets mapped to '_remote_pager_cap' by
> l4_task_map. The '_pager' variable is set via the cpu_session interface,
> like you've stated before. Although, this is normally the "one global
> pager", Genode's design and the Fiasco.OC specific implementation is not
> limited to use only that one pager thread.
> 
> Best regards
> Stefan
> 
>>
>>  
>>
>> Best,
>>
>> Chen
>>
>>
>>
>>
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> 

-- 
Stefan Kalkowski
Genode Labs

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